Implantable are used to provide patients with a constant or programmable dosage or infusion of a drug or any other therapeutic agent. An example of such an implantable drug infusion device currently available is the Medtronic SynchroMed drug pump. Such a device includes a drug reservoir, a peristaltic pump to pump out the drug from the reservoir and a catheter port to transport the pumped out drug from the reservoir via the pump and into the patient. Typically, the drug is provided into the reservoir at a very low pressure and the drug must therefore be forced out of the reservoir and into the patient by a pump. This device requires a battery to power the pump as well as an electronic module to control the pump. Needless to say, because a separate pump battery and electronic module is required, the cost of this device is greater than desired.
An alternative design to an active pumping implantable medical device are devices which do not require the use of a separate pump, but instead rely upon a pressurized reservoir to deliver the drug. An example of such a device includes the Medtronic IsoMed.TM.. The presently available device, however, although requiring low power, does not provide the optimal therapy to the patient. In particular, all such devices currently available feature valves to control the flow of the drug which may only be in a fully open or fully closed position, for example, they are not able to be opened only half-way. The consequences of such a limited option for valve control is that the flow rate of the drug provided to the patient is either completely on or completely off. This causes, not surprisingly, the amount of drug in the patient's blood stream to also vary in a similar fashion, tending to oscillate between a peak and a valley in phase with the opening and closing of the valve. Moreover such valves, besides providing less than optimal drug delivery to the patient, also require a larger amount of energy than is desired. As such, there exists a need for a low power valve which may be used in an implantable medical pump and which provides complete variability in the flow rate through the valve.